add bridge relayer docs

Centrality operated relayer will submit the claims for the depositors
these APIs provide the status of token deposit claims:
- `https://bridge-contracts.nikau.centrality.me/`
- `https://bridge-contracts.rata.centrality.me/`

useful for apps wishing to provide bridging

No file, test, or documentation entry point is named. Start by locating the existing bridge documentation structure, then add the Centrality-operated relayer explanation and the two status API URLs; it is done when apps can understand how claims are submitted and where to check deposit claim status.

Written by the indexing model from the issue text.